Adhesive bonding has been conventionally used as a method of joining composite structures, such as those used in the aerospace industry. Currently, adhesive bonding of composite structures is carried out predominantly by one of three ways: (1) co-curing, (2) co-bonding, and (3) secondary bonding.
“Co-curing” involves joining uncured composite parts by simultaneously curing and bonding, wherein the composite parts are being cured together with the adhesive, resulting in chemical bonding. However, it is difficult to apply this technique to the bonding of uncured prepregs to fabricate large structural parts with complex shapes. Uncured composite materials, e.g. prepregs, are tacky (i.e. sticky to the touch) and lack the rigidity necessary to be self-supporting. As such, uncured composite materials are difficult to handle. For example, it is difficult to assemble and bond uncured composite materials on tools with complex three-dimensional shapes.
“Co-bonding” involves joining a pre-cured composite part to an uncured composite part by adhesive bonding, wherein the adhesive and the uncured composite part are being cured during bonding. The pre-cured composite usually requires an additional surface preparation step prior to adhesive bonding.
“Secondary bonding” is the joining together of pre-cured composite parts by adhesive bonding, wherein only the adhesive is being cured. This bonding method typically requires surface preparation of each previously cured composite part at the bonding surfaces.
Proper surface treatment for co-bonding and secondary bonding is a prerequisite to achieve the highest level of bond line integrity in adhesively bonded structures. Bond line integrity, generally, refers to the overall quality and robustness of the bonded interface. Conventional co-bonding and secondary bonding processes typically include a surface treatment of the composite structures pursuant to the manufacturer's specifications prior to adhesive bonding. Surface treatments include, but are not limited to grit blasting, sanding, peel ply, priming, etc. These surface treatment methods improve adhesion predominantly by mechanical roughening of the surface. The roughened surface allows for better adhesion due to mechanical interlocking at the bonding interface. Such co-bonding or secondary bonding of pre-cured composite structures has a limitation in that the bonding mechanism occurs only through mechanical interlocking with no formation of chemical bonds as in co-cure bonding. Such surface treatments, if performed improperly, could become a source of bond failure during the use of the final bonded structure. Furthermore, in the absence of chemical bond formation at the interface of a composite bonded assembly, the assessment of bond line quality is critical to ensure that proper bonding has occurred. Unfortunately, assessment of bond line quality is often difficult and current techniques known in the art to measure bond line quality are not well suited to measure and evaluate all potential sources of weak bonds.
In the aerospace industry, adhesives are typically used in combination with mechanical fasteners (e.g. rivets, screws, and bolts) to safely and reliably secure structural materials. Rarely are structural adhesives used as the sole mechanism for joining structural parts in an aircraft. Adhesively bonded parts exhibit significant advantages over parts joined by mechanical fasteners including: lighter weight, reduced stress concentrations, durability, lower part count, etc. Despite these benefits, the use of adhesive bonding is limited due, in part, to the difficulty in assessing bond line integrity. Currently, no non-destructive method exists to measure the bond strength of joined parts. The only way to measure the strength of an adhesively bonded joint is to find the ultimate strength, which is obtained by breaking the bond. For obvious reasons, this type of destructive testing is not practical in an industrial manufacturing environment such as the assembly of an aircraft. Moreover, proof testing a large number of specimens to determine the average load capacity of an adhesive does not guarantee that each and every bonded structure will have the expected bond strength.
In order to meet certain aviation certification requirements in countries such as the United States, structural redundancy of primary structures is currently required. Current state-of-the-art bonding methods are not able to satisfy those requirements. Currently, only co-cured structures are certified by the Federal Aviation Administration (FAA) in the United States for primary structures and are used extensively in the aerospace industry. Thus, there remains a need for an adhesive bonding method or technology that can be used in a manufacturing environment as a method of creating reliable and high-strength chemical bonds while providing excellent reproducibility of bond line quality. Furthermore, there remains a need for a bonding method that could satisfy the structural redundancy requirements (e.g. those set out by the FAA in the United States) without adding extra manufacturing steps.